1. Field of the Invention
The present invention relates to a re-generator of an optical clock signal used for an optical repeater in a large optical fiber network.
2. Description of Related Art
Recently, the optical communication network has become larger, using high capacity optical fibers of long lengths. In this kind of optical fiber network, optical repeaters are placed in prescribed positions with a prescribed interval, so as to re-generate light signal. The reason why light signal is re-generated, is to reinforce the light signal as it is weakened by transmission over a long distance. Therefore, it is necessary for the optical re-generator to re-generate optical clock signal accurately.
The conventional optical repeater temporarily transforms a light signal into an electric signal. Then, this repeater re-generates an accurate electric clock signal from this electric signal, and it transforms this re-generated electric clock signal into an optical clock signal once again. In this way, optical clock signal was accurately re-generated.
However, recently, as the high bit rate of a light signal increases, an electric device is not able to catch up with these increases in the high bit rate.
Therefore, it is necessary that a technique to re-generate a light signal directly, rather than transforming the light signal into an electric signal temporarily, be introduced. As an example of this technique, a method of using a mode-locked semiconductor laser has been disclosed (c.f. T. Ono, T. Shimizu, Y. Yano, and H. Yokoyama, OFC'95 Technical Digest, ThL4 P288-289 “Optical clock extraction from 10-Gbit/s data pulses by using monolithic mode-locked laser diodes”).
Operation of a clock re-generator using the mode-locked semiconductor laser disclosed by this document, depends on the angular position of the surface of a polarized wave of the inputted light signal. That is, this kind of semiconductor laser operates very well when the inputted light signal is in horizontal polarized wave state. However, it does not operate well when the inputted light signal is in a vertical polarized wave state.
On the other hand, optical glass fiber (usually, single mode optical fiber), which is used as the medium for distant transmission of a light signal, does not depend on the state of the polarized wave at all. That is, the light signal in an optical fiber travels very well regardless of the state of the light signal, be it a horizontal polarized wave or a vertical polarized wave. However, the state of the polarized wave can be rotated by the optical fiber if laid in a certain place or by the circumstances over the route of the optical fiber.
Therefore, even when a light signal accurately controlled in a horizontal polarized wave state is inputted to an input terminal of an optical fiber, elements of the inputted light signal can be rotated unintentionally. And, at the end of the optical fiber, elements of the horizontal polarized wave and the vertical polarized wave coexist.
As a result, the operation of the re-generator using a mode-locked semiconductor laser, becomes unstable, when the light signal is inputted over a certain length of optical fiber. And, this problem remained unsolved.